Grid electrode for an electric discharge tube



May 24, 1966 K. A. o. HEINZE 8 GRID ELECTRODE FOR AN ELECTRIC DISCHARGE TUBE Filed April 5, 1962 INVENTOR KARL A. O. HEINZE BY z AGENT United States Patent 6 Claims. 61. 313-348) The invention relates to an electric discharge tube, particularly a vacuum discharge tube having a high insulation resistance and comprising at least one control-electrode or grid. With such discharge tubes, which are usually employed for amplification, the control-grids have a negative potential relative to the cathode and the anode. Thus a control of the tube is obtained, which does not require energy.

The electrodes of such tubes are usually centered relative to each other by means of mica or ceramic discs and insulated from each other. Special attention must be given to the insulation of the control-grid, since this grid must not convey current. It has been found that the insulation resistance obtained in the manufacture by the choice of the material of the discs gradually decreases during the use of the tube. This decrease in insulation resistance is due to cathode constituents evaporating from the cathode during operation by thermal evaporation and precipitating in the form of conductive layers on the insulating discs, so that the insulation resistance between the cathode and the grid is reduced. In extreme cases the deposited layers may even render the tube unserviceable.

From measurements it has been found that the total resistance between the stay rods of a control-grid and the further electrodes, particularly the cathode is formed by the resistance of the layer applied by evaporation and the transition resistances between the layer and the metal electrode parts. The transition resistances have a parametric effect. If it is possible to increase one of these resistances, particularly a transition resistance, the insulation can be materially improved.

In accordance with the invention this is achieved in an electric discharge tube by coating with an insulating layer the grid rods, held in insulating discs, substantially throughout their length but at least at the area of their passage through the insulating discs.

It has been found that by coating the grid rods with an insulating layer the insulation resistance is considerably raised and that the transition resistance between the layer applied to the insulating discs in operation and the metal grid rods, in contradistinction to the transition resistances in a non-coated stay member, assumes a comparatively stable value. This effect is due to the fact that the insulation path between metal parts of the electrodes is lengthened by the insulating coating.

It may be expected that the grid rods known from the cathode-ray tube technique, which rods are completely made from ceramic material, also exhibit this insulation effect. With electric discharge tubes of small size, however, ceramic parts cannot be employed, so that this insulation effect has not been assessed before.

The thickness of the insulating coating is chosen so that when the rods are notched prior to winding the grid, the metal rod material is set free on the surface of the notch. The required metallic connection between the grid wire and the metal of the stay rods is thus ensured. Since the surfaces of the notches, viewed from the cathode, are always located outside the stream of evaporated material, an insulation path of adequate length is obtained between the grid rods and the further metal electrode parts.

. 3,253,181 Patented May 24, 1966 The insulating coatings may be made from aluminum which may be applied by a known method by evaporation to the rod surface, after which it is oxidized. A thickness of the oxide layer may be 0.1 or less. A measure for the thickness of the layers used is not only the possibility of removing the layers but also the known quantum-mechanical tunnel effect with respect to electric waves, which effect depends very strongly upon the intermediate insulator and determines the current at the transition areas between the coating applied by evaporation and the metal rod material.

It is known to use alumina layers for insulating purposes. With a known tube an aluminum centering and supporting disc is oxidized to this end, subsequent to its definite formation, on the whole surface, including the surfaces of the holes for passing the rods. Thus an insulating layer is formed between the rods passed tlirough the discs and the material of the centering and fastening disc, but this layer is not provided in addition, since it is required for the aluminum disc to constitute an insulator between the electrodes. When cathode material is deposited on the insulating disc, the high insulation resistance prevailing initially between the insulating disc without a coating of cathode material and the rod material cannot be maintained, since there is no additional insulating layer on the rods in accordance with the invention.

The insulating coating of the rods may, of course, also be made from different insulating material, which does not affect the operation of the tube and which ensures a satisfactory contact between the grid wire and the metal of the rods after notching.

The application of alumina improves the insulation between all electrodes, independently of the relative potential differences. With grids having a negative potential with respect to the cathode, particularly with controlgrids, the blocking effect of semi-conductors may, in addition, be utilized. It has been found that the layers applied to the mica or ceramic discs behave as n-type conductors; In accordance with the invention the grid rods are provided with a p-type conducting coating. Such a coating consists preferably of oxidized nickel.

With negative polarisation of the grid, a pn-junction is formed at the transition between the applied coating and the metal of the supporting member of the electrode, which junction blocks the passage of current between the grid rods and the disc with the applied coating. After a fairly long use tubes having such pn-junctions exhibited an insulation resistance at the control-grid of about a factor higher than the conventional insulation.

The invention will be described more fully with reference to the embodiment shown in the drawing.

The figure shows on an enlarged scale part of an electric discharge tube. A centering disc 1, for example of mica, holds a cathode 3 and rods 5 of the control-grid at the correct relative distance.

The grid rods 5 are provided with a coating 7 of aluminum, the surface of which is oxidized, for example by electrolytic agency The coating 7, operating as an insulating layer owing to the alumina, is interrupted at the places where the grid rods are notched. The interruption is obtained during the notching process. The grid windings 9 are in metallic contact with the rods 5. Viewed from the cathode the notches are located in the shadow of the rods.

The insulating coating 7 is removed from the ends of the rods (at 11), where the connection to the current supply wires (not shown) of the tube pins is established. The insulating coating 7 can be removed at the area concerned by a sharp object, for example a knife. The

places where the insulation is removed are preferably located on the sides of the rods turned away from the cathode.

After a given operational time material of the cathode evaporates and is deposited inter alia on the mica disc 1 in the form of a conductive layer 13. Since a coating 7 is applied to the grid rods 5, the layer applied from the cathode to the mica disc practically can not convey leakage current to the grid rods 5. The high transition resistance of the insulation between the applied conductive layer 13 and the material of the rods counteracts these currents. The leakage currents must flow via the notches 15 of the rods or via the welds of the current supply wires. These paths are, however, considerably longer. Since the bare places of the rods are located on the side of the rods remote from the cathode andhence beyond the stream of evaporating material, so that on this area only a small quantity of material is applied by vaporization, the leakage path is practically blocked.

In a further embodiment of the invention nickel rods are provided by oxidation in moist air at a temperature of about 600 C. with an adhering layer of p-conductive nickel oxide. Between the n-conductive applied layer 13 and the p-conductive nickel oxide layer a p-n-blocking junction is formed, which has a rectifying efiect. At the places where the grid rods 5 are connected to current supply wires by welding and are provided with notches respectively, the p-conductive nickel oxide layer can be removed as readily as the alumina layer.

What is claimed is:

1. An electric discharge tube comprising a cathode, a wire grid, a pair of spaced rods for supporting the wire grid, support means for said cathode and grid support rods, said means being responsive during normal tube operation to boiled-off cathode ions whereby an n-type semi-conductive layer is formed, said support means comprising at least one disc of a semi-conductive material through which the rods extend for supporting said rods, and a layer of p-type semi-conductive material covering at least the surface portion of the rod which extends through the supporting disc whereby a p-n junction is formed at the grid support rods exhibiting high resistivity during negative grid operation.

2. An electric discharge tube comprising a cathode, a wire grid, 21 pair of spaced rods, each having notched portions for supporting the wire grid, support means for said cathode and said grid support rods, said means being responsive during normal tube operation to boiled-off cathode ions whereby an n-type semi-conductive layer is formed, said means comprising at least one disc of insulating material through which the rods extend for supporting said rods, and a layer of p-type semi-conductive material covering the surface of the rod including the portion which extends through the supporting disc except for the notched portions of the rod whereby a p-n junction at the grid support rod exhibiting a high resistivity during negative grid operation.

3. An electric discharge tube as claimed in claim 1 in which the layer of p-type semi-conductive material has a thickness of about one-tenth of a micron.

An electric discharge tube as claimed in claim 1 in which the p-type semi-conductive material is p-type aluminum oxide.

5. An electric discharge tube as claimed in claim 1 in which the p-type semi-conductive layer is p-type nickel oxide.

6. An electric discharge tube comprising a cathode, a Wire grid, a pair of spaced rods supporting the wire grid, support means for said cathode and grid support rods, said means being responsive during normal tube operation to boiled-01f cathode ions whereby an n-type semiconductive layer is formed, said means comprising at least one disc of a semi-conductive material through which the rods extend for supporting said rods, and a layer of p-type semi-conductive material covering at least the surface portion of the rod which extends through the supporting disc whereby a p-n junction is formed at the grid support rods exhibiting high resistivity during negative grid operation, and one end of said grid support rod beyond the emitting area of the cathode being free of the covering layer for providing a terminal connection to the grid.

References Cited by the Examiner UNITED STATES PATENTS 8/1954 Bailin 3l3-350 X 2/1959 Kerstetter 313337 JAMES D. KALLAM, A. S. KATZ, D. O. KRAFT, Assistant Examiners. 

6. AN ELECTRIC DISCHARGE TUBE COMPRISING A CATHODE, A WIRE GRID, A PAIR OF SPACED RODS SUPPORTING THE WIRE GRID, SUPPORT MEANS FOR SAID CATHODE AND GRID SUPPORT RODS, SAID MEANS BEING RESPONSIVE DURING NORMAL TUBE OPERATION TO BOILED-OFF CATHODE IONS WHEREBY AN N-TYPE SEMICONDUCTIVE LAYER IS FORMED, SAID MEANS COMPRISING AT LEAST ONE DISC OF A SEMI-CONDUCTIVE MATERIAL THROUGH WHICH THE RODS EXTEND FOR SUPPORTING SAID RODS, AND A LAYER OF P-TYPE SEMI-CONDUCTIVE MATERIAL COVERING AT LEAST THE SURFACE PORTION OF THE ROD WHICH EXTENDS THROUGH THE SUPPORTING DISC WHEREBY A P-N JUNCTION IS FORMED AT THE GRID SUPPORT RODS EXHIBITING HIGH RESISTIVITY DURING NEGATIVE GRID OPERATION, AND ONE END OF SAID GRID SUPPORT ROD BEYOND THE EMITTING AREA OF THE CATHODE BEING FREE OF THE COVERING LAYER FOR PROVIDING A TERMINAL CONNECTION TO THE GRID. 